February 13, 1987: Rick Cerone was signed as a free agent by the Yankees.[6]

Regular season

The Yankees hit 10 grand slams, the most by an MLB team in 1987.[7] Six of those were hit by Don Mattingly, who set a record for most grand slam home runs in one season with six.[8] His record was matched by Travis Hafner during the 2006 season. Mattingly's Grand Slams in 1987 were also the only six Grand Slams of his career. In addition, Mattingly had tied Dale Long's major league record by hitting home runs in eight consecutive games (record later tied again by Ken Griffey, Jr., of Seattle in 1993), as well as stroking an extra base hit in ten consecutive games. Mattingly had a record 10 home runs during this streak (Long & Griffey had eight of them).
In June 1987, it was reported that Mattingly injured his back during some clubhouse horseplay with pitcher Bob Shirley though both denied this.[9] Nevertheless, he finished with a .327 batting average, 30 home runs, and 115 RBIs, his fourth straight year with at least 110 RBIs.

On July 13, 1987, George Steinbrenner told manager Lou Piniella that the acquisition of Steve Trout would win the Yankees the pennant.[10] Trout never won a game for the Yankees, going 0–4 in 14 games.[11]

August 26, 1987: Ken Patterson and a player to be named later were traded by the Yankees to the Chicago White Sox for Jerry Royster and Mike Soper (minors). The New York Yankees completed the deal by sending Jeff Pries (minors) to the White Sox on September 19.[18]

September 17, 1987: Sherman Obando was signed as an amateur free agent by the Yankees.[19]